The fundamental configuration of a TE.sub.10 rectangular to TE.sub.01 circular waveguide mode launcher as developed by P. Marie' in 1956 is shown perspectively in FIG. 1 as containing three series-coupled sections A, B and C. The first or input section A is shown in perspective in FIG. 2, associated side and end views of which are presented in FIGS. 3 and 4, respectively. This first or input section A into which a TE.sub.10 rectangular mode wave is launched, via a rectangular-shaped input end 11, is formed as a "folded E-plane tee" for converting a TE.sub.10 rectangular mode wave to a TE.sub.20 rectangular mode wave to be emitted at rectangular shaped output end 12.
For this purpose, as shown in FIGS. 2-4, the first conversion section A contains a pair of intersecting rectangular tapered portions 13 and 14. The input or front end 11 of a triangular or tapered portion 13 sits atop a triangular or tapered bottom portion 14 and tapers from an original height 13H along the length of the input section A until it coincides with the upper edge 7 of the wave exiting or output rectangular end 12. The upper surface 15 and the lower surface 16 of triangular shaped section 14 are parallel with one another and separated by a distance equal to the height 14H of section 14. At its output end, the first section A has a width 14W, corresponding to the separation between vertical end walls 18 and 19 which together with upper and lower surfaces 15 and 16 of section 14 form the tapered wall boundaries of section 14. The width 13W of tapered top section 13 represents the separation between parallel side walls 8 and 9 of top section 13 which taper from an initial height 13H to the edge 7 of upper surface 15 at output end 12 with a taper angle .phi.. Side walls 18 and 19 of triangular section 14 form a taper angle .theta. between wall portions 18 and 19 of section 14 and the wall portions 8 and 9 of tapered section 13.
Inserted into the central portion of section A is an impedance matching element 17, which acts to remove a high frequency resonance component and helps match the low end of the band of interest. For purposes of an illustrative example, the mode launcher under consideration is intended to operate in a range of 5.5 to 8.5 GHz.
FIGS. 5A, 5B and 5C illustrate the manner in which an input rectangular mode TE.sub.10 wave is converted to an output rectangular mode TE.sub.20 wave, as the height of the vertical walls 8 and 9 is gradually decreased, while the separation between walls 18 and 19 gradually increases by way of the dual taper of sections 13 and 14. The arrows represent the direction of the electric field vector in the waveguide.
The second section of the mode launcher is shown in perspective in FIG. 6 and in a pair of cross-sectional views of the middle and output ends of the section in FIGS. 7A and 7B, respectively. The arrows represent the direction of the electric field vector in the waveguide. This middle section B is employed to gradually change the TE.sub.20 rectangular mode output wave from the first section A which enters the second section B at open end 21 to an "X" configuration at output end 22, as shown particularly in FIG. 7B.
For this purpose, the input end 21 of second section B has a rectangular-shaped opening defined by a pair of parallel top and bottom walls 27 and 28 and parallel side walls 29 and 30 into which the TE.sub.20 rectangular mode wave from the output of the first section A is coupled. Extending from these walls are four projecting finger portions 23, 24, 25 and 26 forming an "X" cross-sectional shape, each finger portion being a triangular or tapered waveguide section extending from the parallel side walls 29 and 30 at the front end 21 of section B to the output end 22 thereof. This rectangular-to-"X" tapering configuration sets up four orthogonally spaced components of the wave to be aligned with eventual circular waveguide TE.sub.01 mode of the end section C, shown in greater detail in FIG. 8.
As shown therein, to achieve the final circular waveguide output TE.sub.01 mode, a set of four tapered sections 43, 44, 45 and 46 having respective input ends 33, 34, 35 and 36 are aligned with the four "X"-shaped waveguide output ends of the sections 23, 24, 25 and 26 of center section B. Each of sections 43-46 tapers to an output circular shape, as shown in perspective in FIG. 8 and in cross-section in FIG. 9, so that what is launched from the output 50 of the section C is a circular TE.sub.01 mode wave. The circular arrow represents the direction of the electric field vector in the waveguide.
With the sections A, B and C serially interconnected in the manner shown in FIG. 1, the effect of each section is to provide a linear taper from one impedance to the next. If the wavelength distance of each tapered is sufficiently long, each section will be well matched. However, for present day antenna feed operations the Marie' configuration is imperfect. Thus, it has been found necessary to insert an impedance matching element (shown as element 17 in FIGS. 3 and 4) for the purpose of removing the high frequency resonance component and help match the low end of the frequency band of interest. A practical problem that exists in the conventional Marie' configuration is the cost involved in making a precision mandrel to produce the impedance matching element, which is a doubly tapered-wedge shaped piece, and is effectively impossible to machine into a mandrel as would be required. Typically, the mode launcher is electroformed over a mandrel. This means that the impedance matching element could be manufactured separately and later inserted into the section A of the mode launcher. Still, this would be a more expensive procedure than if the impedance matching element could actually be formed as part of the mandrel itself.